The invention relates to fuel assemblies for a nuclear reactor in which a coolant is circulated upwardly through the nuclear core including the assemblies and disposed in the reactor vessel. Each assembly is of the type comprising a bundle of fuel elements each formed by a metal tubular sheath containing a stack of nuclear fuel pellets and closed by gas tight caps. The elements are held in a rigid structure including end pieces connected together by tie rods and spacer grids spaced a part along the tie rods. The elements are distributed at the nodal points of a square network by means of the spacer grids arranged at predetermined intervals.
The invention is particularly suitable, although not exclusively, for assemblies of pressurized water nuclear reactors, called PWRs, having fuel elements with metal sheath made with a zirconium base alloy.
A certain number of separate functions are performed by grids of fuel assemblies of the type which have just been described. On the one hand the grids support laterally and brace the fuel elements containing the fissile material. The grids also ensure structural protection of the assemblies against transverse shocks during handling, during incidents of earthquake origin or operational accidents. On the other hand these grids allow the mixing of the coolant streams circulating upwardly through the core. Such mixing of the coolant is important to allow optimal and continuous regular extraction of the heat released by the fuel elements containing the fissile material, and to avoid local temperature increases within the coolant which could result in a local boiling of the coolant. With an adequate mixing it is thus possible to obtain a general enthalpy increase for the coolant, which is more homogeneously transversely distributed in the core than without mixing, in order to allow a maximum output thermal power dissipation from the core while avoiding local boiling.
Various types of assemblies are already known comprising spacer grids spaced at predetermined intervals and providing the above-mentioned functions. Nuclear assemblies in the prior art in PWR reactors typically comprise grids arranged at regular intervals from top to bottom of the assembly. French document No. 84 18 645 describes assemblies of this type comprising a skeleton or rigid structure having two end pieces connected together by elongated elements such as guide tubes and a plurality of grids spaced apart along the guide tubes and forming support cells for the elements of a bundle of fuel elements, said elements being distributed at the nodal points of a regular network. The grids are of different types and consist in median grids, designed so as to resist lateral shocks and provided with fins to create turbulance in the flow of the coolant along and inside the assembly, lower grids and upper grids ensuring the bracing of the elements and creating a lesser pressure drop for the coolant than the median grids.
This solution solves numerous problems. However, in the particular case of zirconium-based alloy sheath of the fuel elements, applicants have noticed that corrosion appear to a greater extent at the top part of the elements than at their bottom part.
It is known that the local corrosion phenomena depend from the temperature of the coolant and the local thermal power, locally dissipated by each fuel element, and that the temperature of the coolant existing at the top of the core is inevitably greater than at the bottom part of the core. These two factors: the temperature of the coolant and the thermal power locally released by the core, could explain to the technician skilled in the art the differential corrosion between the top and the bottom parts of the fuel elements, but by the same token will convince him that this phenomenon is unavoidable.